In recent years, various electronic apparatuses have been widely used, and those apparatuses include domestic electric appliances such as televisions, computers such as personal computers, mobile communication apparatuses such as mobile telephones and medical equipment. Unnecessary electromagnetic waves released from these electronic apparatuses give influences to other electronic apparatuses to cause adverse effects such as malfunction. For this reason, an electromagnetic interference suppressor, which removes or shields those unnecessary electromagnetic waves, is used in such electronic apparatuses.
Here, recently, the above-mentioned electronic apparatuses have been rapidly improved to provide high-speed, light-weight and thin structures, and miniaturized products, so that the packaging density of electronic parts to a circuit has become extremely higher. For this reason, along with an increase in electromagnetic noise caused by electromagnetic interference between parts as well as between circuit substrates, there has been a higher possibility of occurrence of electromagnetic interference due to unnecessary electromagnetic waves also between parts as well as between circuit substrates within an electronic apparatus.
Moreover, mobile terminals (for example, mobile telephones, IC cards, and RF-ID systems such as tags) having an IC tag function that carries out radio communications by using electromagnetic waves centered on a 13.56 MHz band have been put into a practical use. In one of the applications, a receiving-use loop antenna is placed in a housing of a small-size, thin-type mobile telephone, and in this case, when a metal housing or the inner face of the housing that has been subjected to a conductivity-applying treatment, such as plating, for the purpose of electromagnetic wave shielding is located closely to the loop antenna, lines of a magnetic force of a magnetic field, generated around the loop antenna upon transmitting or receiving operation, travel in parallel with the metal surface to generate an eddy current on the metal surface to cause losses. Moreover, a magnetic field, generated by the eddy current is formed in such a direction as to cancel the first magnetic field (forming an anti-magnetic field), and since the resonance frequency is shifted, the magnetic field in the frequency to be used for communications attenuates greatly, resulting in a phenomenon that shortens the communication distance drastically.
One of the countermeasures to communication disturbance due to metal located near the loop antenna in radio communications caused by a magnetic coupling is to place a magnetic shield sheet (magnetic sheet) between the loop antenna and the housing. With respect to the magnetic shield sheet, for example, a sheet has been proposed in which the numeric value of the real part (μ′) of a complex specific magnetic permeability is high at 13.56 MHz (which collects magnetic fluxes easily) and the numeric value of the imaginary part (μ″) thereof is low (which hardly converts collected magnetic fluxes into heat). An electromagnetic interference suppressor which the present invention refers to is used for this magnetic shield sheet.
Patent Document 1 has disclosed that one of the countermeasures to suppress electromagnetic interference is to place a sheet-state electromagnetic interference suppressor formed by dispersing soft magnetic powder in a binder near an electronic part or circuit.
When the electromagnetic interference suppressor of this kind is used, a sheet having a high permeability in a range from several tens of MHz to several GHz is required. In order to achieve a high permeability, it has been proposed that soft magnetic powder having particles of not a spherical shape but a flattened shape is used and that the particles of this flattened soft magnetic powder are aligned along the surface of the electromagnetic interference suppressor sheet (Patent Document 2).
In order to easily achieve such an alignment, it is preferable to use a material having high flowability as a matrix material. For example, Patent Document 2 has disclosed a technique in which a magnetic paint formed by dissolving flattened soft magnetic powder and a polymer binder in an organic solvent is applied onto a peeling support member by using a doctor blade method, and dried thereon as a sheet. However, in the case when this processing method is used, since upon drying the solvent in the magnetic paint generates foam, a problem arises in that a large number of pores are formed in the sheet. When a large number of pores are generated, the electromagnetic interference suppressing effect is lowered drastically. Therefore, there have been strong demands for a method which can fill the soft magnetic powder with high density, while suppressing the generation of pores as little as possible.
On the other hand, Patent Document 3 has disclosed a method of producing a composite magnetic material, which relates to a method in which a mixed matter, obtained by mixing and kneading flattened magnetic powder and a binder, is formed into a sheet by a predetermined method, and allows the binder to contain a vinyl chloride-based resin having a glass transition point of 50° C. or more. In this method, however, in order to allow the resulting composite magnetic material to have high density, another pressing process is required in which the sheet, which has been film-formed and from which the solvent has been removed, is pressed by a rolling apparatus using a press and rollers.    [Patent Document 1] JP-A No. 7-212079    [Patent Document 2] JP-A No. 2003-229694    [Patent Document 3] JP-A No. 2001-126910